Generally, rear derailleurs use with a rear side multistage sprocket assembly are provided with a linkage mechanism comprising a fixing member fixed to a fork end of the bicycle frame, a pair of linkage members, and a movable member having a chain guide. The linkage mechanism is disposed axially outwardly of a smaller diameter sprocket at the multistage sprocket assembly, and the fixing member is fixed to the fork end. The movable member is moved in reciprocation by operating a control wire, and a driving chain engaging with pulleys at the chain guide is shifted to one sprocket at the multistage sprocket assembly, thereby changing the bicycle speed at a predetermined speed changing stage.
Problems exist where the fixing member at the linkage member is fixed to the fork end at the bicycle frame. Since the linkage mechanism projects outwardly with respect to the bicycle frame and no guard is provided for guarding the linkage mechanism, the linkage mechanism is subject to the danger of colliding with the ground or a foreign object, for example, when the bicycle falls down, and is subjected to a force or pressure greater than a resistance against speed change. In this situation, the fixing member particularly is subject to being plastically deformed axially inwardly of the multistage sprocket assembly with respect to the frame. As a result, a problem is created in that the chain guide shifts from the proper position with respect to the multistage sprocket assembly.
When the durability of the fixing member is raised, the plastic deformation thereof caused by a force or pressure greater than the resistance against speed change is reduced, but a problem is created in that the fork end at the bicycle frame is plastically deformed. Thus, the aforesaid problems still remain unsolved.